The control techniques presently employed for operating automobile power door locks, power windows and power seats fall into three categories. The first technique which may be characterized as local control/direct switching, is employed when the input switch and the device it controls are in the same area of the vehicle. For example, the input control switch and actuator motor for the driver's power window are both located inside the driver's door. The second technique, remote control/direct switching, is employed when the input control switch and the device it controls are located in different areas of the vehicle. For example, the input switches located in the driver's door remotely control the window motors in the other three doors. These two techniques require the use of high current input switches and large gauge cables to distribute the power. The third technique, remote control/indirect switching, is generally employed when a more complex control system is required. For example, the power door lock system employs input switches to control a relay which in turn actuates the motors which are located in each of the four doors. This technique permits the use of low current input switches and small gauge control wires which minimizes the large gauge cables required to distribute power.
A multiplex control system is a more refined form of remote control/indirect switching that reduces to a minimum the long length wires required between the input switches and the devices being controlled. In a multiplex control system the input switch information is electronically converted so that it can be sent over a single wire data link to a different area of the vehicle. A receiver then electronically interprets this data and directs it to the proper power switch which in turn instructs the actuator motor to operate. By using a multiplex control system the number of wires, and the conductor crosssectional area, can be substantially reduced. While this reduction in conductors represents considerable improvement, there are other important requirements, such as producibility, reliability, performance and cost which have not been entirely fulfilled in prior art systems which have made them unacceptable for automobile application. One of the problems associated with prior electronic multiplexing systems is their susceptibility to the noisy environment of the automobile. Prior approaches to solving that problem have been attempts to isolate or immunize the system from the environment. Also, prior art systems employ complex digital electronics such as microprocessors at the transmitting and receiving end of the multiplex system. In order to provide a precise time base for serial data communication a crystal reference is usually required with its attendant expense. Furthermore, a precise supply voltage is required which implies external regulator and filter circuitry. Often buffering electronics is required on the inputs and outputs. Such systems thus require printed circuit boards and housings for mounting the various components thereby increasing the space requirements for incorporating such a system in the automobile.
With the foregoing in mind, it is an object of the present invention to provide a multiplex system which requires no special filtering techniques in order to operate reliably in the automotive electrical environment.
It is another object of the present invention to provide a multiplex system utilizing monolithic integrated circuits of relatively simple design, requiring a minimum amount of discrete components and no external filtering thereby eliminating the need for circuit boards and the attendant space requirements.